Process for producing saturated aliphatic carboxylic acid amide

ABSTRACT

A process for producing a saturated aliphatic carboxylic acid amide, comprising reacting a saturated aliphatic carboxylic acid with ammonia to thereby obtain a saturated aliphatic ammonium carboxylate and subjecting the saturated aliphatic ammonium carboxylate to a dehydration reaction for obtaining a saturated aliphatic carboxylic acid amide, wherein the dehydration reaction of the saturated aliphatic ammonium carboxylate is conducted while supplying water to a reaction system in which the dehydration reaction is carried out. Preferably, water is continuously supplied to the reaction system so that the production of the saturated aliphatic carboxylic acid amide is carried out in a continuous manner and the amount of water present in a steady state ranges from 20 to 70 mol per 100 mol of a total of the saturated aliphatic carboxylic acid, the ammonia, the saturated aliphatic ammonium carboxylate, the saturated aliphatic carboxylic acid amide and the water. This process enables simplification of the production steps, ensures high operation efficiency, lowers production facility constructing and operating costs, reduces the amount of by-products and enables producing a high-purity carboxylic acid amide at a high selectivity, at a high yield and on an industrial scale.

This application is an application filed under 35 U.S.C. §111(a)claiming benefit pursuant to 35 U.S.C. §119(e)(1) of the filing date ofthe Provisional Application No. 60/047,101, filed May 19, 1997, pursuantto 35 U.S.C. §111(b).

FIELD OF THE INVENTION

The present invention relates to a process for producing a saturatedaliphatic carboxylic acid amide (hereinafter often referred to simply as“carboxylic acid amide”) which is widely used as solvents, solubilizingagents, plasticizer stabilizing agents, dyes, drugs and materials foruse in organic syntheses. More particularly, the present invention isconcerned with a process for effectively producing a highly purifiedcarboxylic acid amide with the formation of by-products suppressed, inwhich a dehydration reaction of an ammonium salt obtained fromcorresponding saturated aliphatic carboxylic acid (hereinafter oftenreferred to simply as “carboxylic acid”) and ammonia is conducted whilesupplying water to the reaction system to thereby obtain a carboxylicacid amide.

Further, the present invention relates to a process in which part ofwater is separated and removed from a mixture of a saturated aliphaticammonium carboxylate and water by distillation. More particularly, thepresent invention is concerned with a process for efficiently recoveringa saturated aliphatic ammonium carboxylate by adding a saturatedaliphatic carboxylic acid to a mixture of a saturated aliphatic ammoniumcarboxylate and water and thereafter conducting a distillation.

BACKGROUND OF THE INVENTION

It is known that the carboxylic acid amide can be produced bysynthesizing an ammonium carboxylate salt from a carboxylic acid andammonia and subjecting the salt to a dehydration reaction.

The carboxylic acid amide can be produced by a batch process or acontinuous process. In an industrial process, conditions for separatingand purifying the carboxylic acid amide as a product and conditions forremoving and recycling by-products and unreacted raw materials aregreatly influenced by, for example, the types and properties of desiredcarboxylic acid amide, individual starting materials and by-products. Inparticular, the cost and availability of raw materials, the final yieldof desired product after separation and purification, the simplicity ofentire production steps including a separation/purification system, thecost of production facility construction, the operation efficiency, theutility cost, etc. are extremely important factors, and an industrialprocess must be decided by collectively judging all of these. When thecarboxylic acid amide is produced by, for example, a batch process,conventionally, the reaction temperature is set at 150 to 220° C. andthe reaction time at 12 to 30 hr. and water formed as a by-product bythe reaction is removed from the top of the reactor as a low boilingpoint fraction.

When the carboxylic acid amide is produced by this process, theconversion of the ammonium carboxylate can be 90% or higher. However,by-products such as nitrile compounds and carboxylic acid amidedimerization products are formed in relatively large amounts, so thatthe selectivity of carboxylic acid amide is as low as 90 to 95% and thefinal yield of carboxylic acid amide is also not satisfactory. Moreover,the production of carboxylic acid amide by the batch reaction prolongsthe processing time and increases the formation of by-products, so thatthe above process cannot be regarded as an industrially advantageousprocess from a judgement of its productivity and economy.

For example, U.K. Patent No. 935,391 and Japanese Patent Laid-openPublication No. 57(1982)-38754 disclose a continuous process forproducing a carboxylic acid amide, in which a carboxylic acid or anammonium carboxylate is continuously fed into a reaction column throughits top or its middle while ammonia is continuously fed into thereaction column through its lower part to thereby obtain a carboxylicacid amide.

In this process, with respect to reaction starting materials, ammonia isused in excess of the carboxylic acid. The excess ammonia is distilledout from the top of the reaction column together with water. Forcondensing the fraction from the top of the reaction column to therebyattain a recovery, a cooling medium of extremely low temperature must beused. Further, it is needed to install a separator for distilling wateroff recovered aqueous ammonia and install an absorption column forrecovering ammonia gas obtained by this separation. Therefore, the aboveprocess cannot be regarded as a process which is satisfactory from theviewpoint of economy and operation efficiency.

In the process described in these published specifications, much ofammonia gas formed by a a decomposition reaction of ammonium carboxylateis distilled outside the system from the top of the reaction column.Therefore, the above process involves problems such that not only is theyield of carboxylic acid amide relative to the amount of ammonia addedas a starting material unsatisfactory but also by-products such asnitrile compounds and carboxylic acid amide dimerization products(dimerized amide of carboxylic acid) are formed in relatively largeamounts.

Therefore, there is a demand for the development of a process forproducing a carboxylic acid amide, which enables producing a highlypurified carboxylic acid amide at a high yield from a carboxylic acidand ammonia as raw materials.

In addition to the above processes, a process for producing a carboxylicacid amide has been proposed, in which the carboxylic acid amide isproduced from an ammonium carboxylate in the presence of a dehydrationcatalyst. In this process, molybdenum oxide, an alkyltin catalyst, amixture of silica gel and alumina or a titanium tetrachloride catalystis used as the dehydration catalyst.

In this process, the dehydration reaction can be carried out atrelatively low temperatures, so that not only can the formation ofby-products such as nitrile compounds and carboxylic acid amidedimerization products be suppressed but also the reaction time can beshortened. However, when the carboxylic acid amide is produced with theuse of the catalyst, it is apprehended that the catalyst be dissolved orotherwise contained in the obtained carboxylic acid amide and, hence, astep of separating or otherwise removing the catalyst from thecarboxylic acid amide after the completion of the reactions becomesrequisite. If costs of catalyst production and catalyst removal incurredby the use of the catalyst, etc. are taken into account, this processalso cannot be regarded as a satisfactory process.

The applicant of the present patent application proposed in JapanesePatent Laid-open Publication No. 9(1997)-157233 a process for producinga carboxylic acid amide represented by the formula: RCONH₂ (wherein Rrepresents a saturated alkyl group having 1 to 4 carbon atoms),comprising:

a first step comprising supplying, to a reactor consisting of arectifying tower and a reaction vessel, a carboxylic acid represented bythe formula RCOOH (wherein R is as defined above) and an ammonium saltthereof (provided that, in the total supply, the molar amount of thecarboxylic acid is in the range of from 0.1 to 2.0 times that of theammonium salt), which are fresh or recycled from the following secondand third steps; conducting a dehydration reaction of the ammonium saltat 130 to 200° C.; and distilling mixture components consisting offormed water and the carboxylic acid and/or unreacted ammonium salt offthe top of the rectifying tower to thereby obtain components containingthe carboxylic acid amide from the reaction vessel;

a second step comprising distilling the components containing carboxylicacid amide obtained in the first step to thereby purify the carboxylicacid amide; and

a third step comprising distilling water off the mixture componentsconsisting of water and carboxylic acid and/or unreacted ammonium saltrecovered in the first step.

This process enables producing a highly purified saturated lower alkylcarboxylic acid amide simply at a high yield with an industrialadvantage, as compared with the above prior art processes. However, thisprocess has room for improvement because by-products such asdicarboxylic acid amides and carboxylic acid salt of amidine are formedin a considerable amount in the dehydration reaction of the ammoniumcarboxylate for producing the carboxylic acid amide.

On the other hand, the obtained reaction mixtures containing carboxylicacid amide and water, including that obtained by the carboxylic acidamide producing process described in the above literature, generallyalso contain unreacted raw material ammonium carboxylate, etc. Thereaction mixtures can be fractionated into low boiling point componentssuch as ammonium carboxylate and water and high boiling point componentssuch as carboxylic acid amide by a purification according to thecustomary distillation method. Thus, a carboxylic acid amide ofrelatively high purity can be obtained.

However, the lower aliphatic carboxylic acid amide has a relatively highboiling point and its thermal stability is poor at a temperature zonenear the boiling point, so that, for example, the formation ofimpurities (by-products) is likely to occur in the purification stage.Main incidentally formed impurities include dicarboxylic acid amideswhich are amides resulting from dimerization of the carboxylic acid. Thedicarboxylic acid amide and the carboxylic acid amide have approximatelyidentical boiling points, so that these are obtained as high boilingpoint components. It is extremely difficult to purify the carboxylicacid amide by separating the same from the dicarboxylic acid amide withthe use of customary distillation and purification means.

Further, it is known that this dicarboxylic acid amide is decomposedinto a carboxylic acid and a nitrile compound when exposed to hightemperature.

When it is intended to produce, for example, an N-vinyl carboxylic acidamide (one of useful vinyl monomers) by reacting a starting material ofa carboxylic acid amide containing the above dicarboxylic acid amide asimpurity with acetylene or the like at high temperatures, adecomposition reaction of the dicarboxylic acid amide simultaneouslyoccurs in the production stage with the result that a carboxylic acidand a nitrile compound are formed. The formed carboxylic acid causes adeterioration of the reaction mixture containing the N-vinyl carboxylicacid amide, thereby inducing, for example, formation of polymers. As aresult, troubles occur such as clogging of lines for production of theN-vinyl carboxylic acid amide, and the yield of the N-vinyl carboxylicacid amide drops. Therefore, it is not desirable to directly use thecarboxylic acid amide containing the dicarboxylic acid amide as astarting material in the synthesis of the N-vinyl carboxylic acid amide.

The purification method combining the distillation method and therecrystallization method is generally known as providing means forobtaining a highly purified carboxylic acid amide not containing anydicarboxylic acid amide, which is suitably used as a starting materialin the synthesis of, for example, an N-vinyl carboxylic acid amide. Theuse of the recrystallization method in combination with the distillationmethod provides An effective means for removing the dicarboxylic acidamide whose separation is difficult only by the distillation method.However, this recrystallization method generally employs an aqueoussolution of alcohol in which the solubility of carboxylic acid amide isrelatively high as a recrystallization solvent, so that, after therecrystallization, the carboxylic acid amide is present at a relativelyhigh concentration in the mother liquor. Further, in this method, theyield of the carboxylic acid amide is so low that the mother liquor mustbe reprocessed for enhancing the yield. Still further, this methodrequires a drying step for removing the recrystallization solvent, whichis disadvantageous from the viewpoint of cost.

For using the carboxylic acid amide containing dicarboxylic acid amideas a starting material in the synthesis of, for example, an N-vinylcarboxylic acid amide, it is required that some pretreatment be carriedout to convert the dicarboxylic acid amide to components which are notdetrimental to the production of an N-vinyl carboxylic acid amide, etc.

As mentioned above, the reaction mixture containing the carboxylic acidamide and water can be fractionated into high boiling point componentscomposed mainly of the carboxylic acid amide and low boiling pointcomponents composed mainly of an ammonium carboxylate and water by thedistillation method.

If recovered, the above ammonium carboxylate can be reutilized as a rawmaterial for producing the carboxylic acid amide, and itself is usefulas, for example, a dye buffer, an organic synthesis buffer, a rawmaterial for drug or a raw material for organic synthesis.

It is important to remove water from the low boiling point componentscontaining the ammonium carboxylate and water to thereby recover andreutilize the ammonium carboxylate from the viewpoint that the quantityof waste is reduced and that the carboxylic acid amide is produced at ahigh yield.

The removal of water from the low boiling point components (mixture)containing the ammonium carboxylate and water can be performed by therecrystallization method. The water separation according to therecrystallization method is performed by either cooling to below thecrystallization temperature of the mixture or adding an to the ammoniumcarboxylate. In these recrystallization organic solvent such as analcohol which is a poor solvent methods, the solubility of ammoniumcarboxylate in water is so high that the ammonium carboxylate is left inrelatively high concentration in the residual liquid remaining after therecovery of ammonium carboxylate. For obtaining the ammonium carboxylateat a high yield, it is needed to further recover the ammoniumcarboxylate from the residual liquid. In the cooling crystallizationmethod, when the water concentration (water content) is high, measuresmust be taken such that the crystallization temperature is set at anextremely low or that an organic solvent is added. When an organicsolvent is used, it is additionally needed to implement a drying stepfor removing the organic solvent adhering to obtained crystals ofammonium carboxylate and a step for recycling the organic solvent. Fromthe viewpoint of facility cost, productivity, etc. of these steps, thepurification according to the cooling crystallization of ammoniumcarboxylate with the use of the organic solvent cannot be regarded as aneffective purification method available in an industrial process.

On the other hand, in the method in which water is removed from the lowboiling point components containing the ammonium carboxylate and waterwith the use of conventional distilling apparatus, the ammoniumcarboxylate is decomposed into a carboxylic acid and ammonia even atrelatively low temperatures because its thermal stability is poor. Theammonia formed by the decomposition is distilled together with water offthe top of the apparatus. When it is intended to recover the ammonia, anoperation for further separating water from a fraction containingammonia and water is needed. Thus, it is an nonefficient separatingmethod and is not suitable to an industrial process. Therefore, there isa demand for the development of a simple economic technology forrecovering the ammonium carboxylate from the low boiling pointcomponents containing the ammonium carboxylate and water.

As mentioned above, the carboxylic acid amide is produced by thedehydration reaction of the ammonium carboxylate. In the conventionalmethod, side reactions occur and products of such side reactions(impurities) are concentrated and contained in the residue resultingfrom separation of the carboxylic acid amide from the reaction mixture.Although containing impurities much, this residue is composed mainly ofthe carboxylic acid amide. From the industrial point of view, it isdesired to attain further recovery of the carboxylic acid amide from theabove residue or attain effective utilization thereof, for example,recycling to the production stage.

Therefore, there is also a demand for the development of a process forproducing a carboxylic acid amide, in which an effective utilization canbe made of the residue resulting from recovery of the carboxylic acidamide from the carboxylic acid amide synthesizing reaction mixture andin which side reaction products that are detrimental to the quality ofthe carboxylic acid amide are not accumulated.

OBJECT OF THE INVENTION

The present invention has been made with a view toward resolving theabove problems of the prior art. An object of the present invention isto provide a process for producing a carboxylic acid amide, whichenables simplification of the production steps, ensures excellentoperation efficiency, lowers production facility constructing andoperating costs, reduces the amount of by-products and enables producinga highly purified carboxylic acid amide at a high selectivity, at a highyield and on an industrial scale.

Another object of the present invention is to provide a process forproducing a carboxylic acid amide, which can attain an effectiveutilization of the residue resulting from recovery of the carboxylicacid amide and avoids accumulation, in the reaction system, of sidereaction products that are detrimental to the production of thecarboxylic acid amide.

An additional object of the present invention is to provide a processfor purifying a carboxylic acid amide, in which a crude carboxylic acidamide containing a dicarboxylic acid amide is purified to thereby enableproducing a highly purified carboxylic acid amide suitably usable as araw material in the synthesis of an N-vinyl carboxylic acid amidesimply, at a high yield and with an industrial advantage.

A further object of the present invention is to provide a process forefficiently producing a saturated aliphatic ammonium carboxylate byremoving water from a mixture containing the ammonium carboxylate andwater.

SUMMARY OF THE INVENTION

The process for producing a saturated aliphatic carboxylic acid amideaccording to the present invention comprises reacting a saturatedaliphatic carboxylic acid with ammonia to thereby obtain a saturatedaliphatic ammonium carboxylate and subjecting the saturated aliphaticammonium carboxylate to a dehydration reaction for obtaining a saturatedaliphatic carboxylic acid amide,

wherein the dehydration reaction of the saturated aliphatic ammoniumcarboxylate is conducted while supplying water to a reaction system inwhich the dehydration reaction is being carried out.

In the present invention, preferably, water is supplied to the reactionsystem so that the production of the saturated aliphatic carboxylic acidamide is carried out in a continuous manner, and the amount of waterpresent in a steady state ranges from 20 to 70 mol per 100 mol of atotal of the saturated aliphatic carboxylic acid, the ammonia, thesaturated aliphatic ammonium carboxylate, the saturated aliphaticcarboxylic acid amide and the water.

The above process for producing a saturated aliphatic carboxylic acidamide according to the present invention preferably comprises:

[I] a first step comprising:

feeding to a reaction vessel, as raw materials,

(a) the saturated aliphatic carboxylic acid, ammonia and water, and/or

(b) the saturated aliphatic ammonium carboxylate and water,

and conducting the dehydration reaction of the saturated aliphaticammonium carboxylate in the presence of water to thereby form thesaturated aliphatic carboxylic acid amide and water;

[II] a second step comprising distilling (preferably under reducedpressure) a reaction mixture obtained in the first step containing thesaturated aliphatic carboxylic acid amide and water, in a firstrectifying tower so that the reaction mixture is separated into lowboiling point components containing the saturated aliphatic ammoniumcarboxylate and water and high boiling point components containing thesaturated aliphatic carboxylic acid amide, thereby obtaining thesaturated aliphatic carboxylic acid amide; and

[III] a third step comprising distilling off part of water from the lowboiling point components obtained in the second step in a secondrectifying tower to thereby obtain an aqueous solution of saturatedaliphatic ammonium carboxylate and feeding the aqueous solution to thefirst step.

In the above processes of the present invention without exception, it ispreferred that the dehydration reaction of the saturated aliphaticammonium carboxylate be conducted at 130 to 250° C. under a pressure of2 to 20 kgf/cm².

In the present invention, still preferably, the high boiling pointcomponents obtained in the second step, containing the saturatedaliphatic carboxylic acid amide in high concentration, are furtherrectified to thereby separate the saturated aliphatic carboxylic acidamide from the high boiling point components to obtain a residue, andwater is added to the residue to thereby hydrolyze side reactionproducts contained in the residue so that a mixture containing thesaturated aliphatic carboxylic acid amide and the saturated aliphaticammonium carboxylate is obtained, the above mixture recycled to thefirst step or the second step.

Also, it is preferred in the present invention that an alcohol of theformula: R²OH (wherein R² represents an alkyl group having 1 to 4 carbonatoms) be added to the high boiling point components obtained in thesecond step, containing the saturated aliphatic carboxylic acid amide inhigh concentration, to thereby carry out an alcoholysis of by-productscontained in the high boiling point components, so that the saturatedaliphatic carboxylic acid amide is obtained.

Further, it is preferred in the present invention that an alcohol of theformula: R²OH (wherein R² represents an alkyl group having 1 to 4 carbonatoms) be added to a distillate obtained by further rectifying the highboiling point components obtained in the second step, containing thesaturated aliphatic carboxylic-acid amide in high concentration so as toseparate the saturated aliphatic carboxylic acid amide, thereby carryingout an alcoholysis of by-products contained in the distillate with theresult that the saturated aliphatic carboxylic acid amide is obtained.

In the present invention, the by-product is generally the saturatedaliphatic dicarboxylic acid amide. The alcohol is preferably methanol.

Moreover, in the third step of the present invention, it is preferredthat a saturated aliphatic carboxylic acid be added to the low boilingpoint components obtained in the second step, containing the saturatedaliphatic ammonium carboxylate and water, and distilled in the presenceof 0.1 to 10 mol of the saturated aliphatic carboxylic acid per mol ofthe saturated aliphatic ammonium carboxylate to thereby separate water.

In the present invention, preferably, the saturated aliphatic carboxylicacid is represented by the formula: R¹COOH (wherein R¹ represents analkyl group having 1 to 4 carbon atoms), the saturated aliphaticammonium carboxylate is represented by the formula: R¹COONH₄ (wherein R¹is as defined above) and the saturated aliphatic carboxylic acid amideis represented by the formula: R¹CONH₂ (wherein R¹ is as defined above).Still preferably, the saturated aliphatic carboxylic acid is aceticacid, the saturated aliphatic ammonium carboxylate is ammonium acetateand the saturated aliphatic carboxylic acid amide is acetamide.

The above process for producing a saturated aliphatic carboxylic acidamide according to the present invention enables simplification of theproduction steps, ensures excellent operation efficiency, lowersproduction facility constructing and operating costs, reduces the amountof by-products and enables producing a highly purified carboxylic acidamide at a high selectivity, at a high yield and on an industrial scale.

The present invention also provides a process for producing a carboxylicacid amide, which can attain an effective utilization of the residueresulting from recovery of the carboxylic acid amide and avoidsaccumulation, in the reaction system, of side reaction products that aredetrimental to the production of the carboxylic acid amide.

Further, the present invention provides a process for purifying acarboxylic acid amide, in which a crude carboxylic acid amide containinga dicarboxylic acid amide is purified to thereby enable producing ahighly purified carboxylic acid amide suitably usable as a raw materialin the synthesis of an N-vinyl carboxylic acid amide simply, at a highyield and with an industrial advantage.

Still further, the present invention provides a process for efficientlyproducing a saturated aliphatic ammonium carboxylate by removing waterfrom a mixture containing the ammonium carboxylate and water.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic chart showing a preferred mode of the process forproducing a saturated aliphatic carboxylic acid amide according to thepresent invention; and

FIG. 2 is a schematic chart showing the conventional process forproducing a saturated aliphatic carboxylic acid amide as described inComparative Example E1, in which Ac is an abbreviation for a group ofthe formula CH₃CO—.

DETAILED DESCRIPTION OF THE INVENTION

The process for producing a saturated aliphatic carboxylic acid amideaccording to the present invention will be described in detail below.

Referring to the following formula, in the process for producing asaturated aliphatic carboxylic acid amide according to the presentinvention, which comprises reacting a saturated aliphatic carboxylicacid (e) with ammonia to thereby obtain a saturated aliphatic ammoniumcarboxylate (f) and subjecting the saturated aliphatic ammoniumcarboxylate to a dehydration reaction for obtaining a saturatedaliphatic carboxylic acid amide (g),

the dehydration reaction of the saturated aliphatic ammonium carboxylate(f) is conducted while supplying water to a reaction system in which thedehydration reaction is carried out.

wherein R¹ represents an alkyl group, preferably, an alkyl group having1 to 4 carbon atoms. Particularly preferred is methyl.

Conducting the above dehydration reaction while supplying water to thereaction system realizes hydrolyses of side reaction products such asdicarboxylic acid amides represented by the formula (R¹CO)₂NH (whereinR¹ is as defined above) and carboxylic acid salts of amidine (h) of theformula:

wherein R¹ is as defined above. The above hydrolyses of the by-productsconvert the dicarboxylic acid amide as one of the by-products tocorresponding carboxylic acid amide, carboxylic acid and the like andconverts the amidine carboxylic acid salt to corresponding carboxylicacid amide, ammonium carboxylate and the like, so that a mixturecomposed mainly of the carboxylic acid amide and ammonium carboxylate(including water supplied for hydrolysis) can be obtained. Therefore,the amount of the above by-products whose adverse effect on the reactionsystem is apprehended can be reduced and accordingly the synthesis ofthe carboxylic acid amide can be performed at a high selectivity.

In this invention, water is supplied, preferably, continuously suppliedto the reaction system so that the production of the saturated aliphaticcarboxylic acid amide is carried out, and the amount of water present ina steady state ranges from 20 to 70 mol, preferably, 35 to 55 mol per100 mol of a total of the saturated aliphatic carboxylic acid, theammonia, the saturated aliphatic ammonium carboxylate, the saturatedaliphatic carboxylic acid amide and the water.

It is preferred that the above dehydration reaction of the saturatedaliphatic ammonium carboxylate be conducted at 130 to 250° C.,especially, 160 to 200° C. under a pressure of 2 to 20 kgf/cm²,especially, 4 to 10 kgf/cm². When the reaction temperature is lower than130° C., the conversion of the ammonium salt is not satisfactorily highand the processing time is likely to prolong. On the other hand, whenthe reaction temperature is higher than 250° C., the formation ofby-products is increased and the pressure of the reaction system becomeshigh to thereby necessitate an expensive pressure reaction vessel. Thisis disadvantageous from the viewpoint of cost. Although depending onreaction conditions such as reaction temperature, the reaction timegenerally ranges from 1 to 10 hr in terms of residence time.

The above saturated aliphatic carboxylic acid amide (carboxylic acidamide) is represented by the formula: R¹CONH₂ (wherein R¹ represents analkyl group having 1 to 4 carbon atoms), which is, for example,acetamide, propionamice, iso-butyramide, n-butyramide, pivalamideiso-valeramide or n-valeramide, preferably, acetamide.

The carboxylic acid for use in the reaction or carboxylic acid for usein the production of the ammonium salt thereof is carboxylic acidcorresponding to the above carboxylic acid amide, which is representedby the formula: R¹COOH (wherein R¹ represents an alkyl group having 1 to4 carbon atoms), which is, for example, acetic acid, propionic acid,iso-butyric acid, n-butyric acid, pivalic acid, iso-valeric acid orn-valeric acid.

The above process for producing the saturated aliphatic carboxylic acidamide (carboxylic acid amide) according to the present invention will bedescribed in greater detail below with reference to the drawings.

FIG. 1 is a schematic chart showing a preferred mode of the process forproducing a saturated aliphatic carboxylic acid amide according to thepresent invention.

[First Step]

The process for producing a carboxylic acid amide as shown in FIG. 1comprises first, second and third steps. In the first step [I], rawmaterials, specifically,

(a) the saturated aliphatic carboxylic acid represented by the formula:R¹COOH (wherein R¹ is as defined above), ammonia and water, and/or

(b) the saturated aliphatic ammonium carboxylate represented by theformula: R¹COONH₄ (wherein R¹ is as defined above) and water,

is fed into a reaction vessel (1) and the dehydration reaction of thesaturated aliphatic ammonium carboxylate is conducted in the presence ofwater to thereby form the saturated aliphatic carboxylic acid amide andwater.

In this first step, the dehydration reaction is conducted at 130 to 250°C., preferably, 160 to 200° C. Although depending on reaction conditionssuch as reaction temperature, the reaction time generally ranges from 1to 10 hr in terms of residence time.

With respect to the molar ratio of raw materials fed into the reaction,it is desired that the carboxylic acid (including the carboxylic acidportion of the ammonium salt recycled from the second or third step) beused in an amount of 0.1 to 10 mol, especially, 0.5 to 2 mol and, stillespecially, 0.8 to 1.2 mol per mol of the ammonia (including the ammoniaportion of the ammonium salt recycled from the second or third step).

The addition of the carboxylic acid in amounts larger than or smallerthan the above range has little influence on the reaction performance,etc. However, the amount of recycled ammonia or carboxylic acid isincreased, so that it is wasteful from the viewpoint of equipment andenergy.

When the dehydration reaction is carried out under such conditions thatthe molar ratio of supplied carboxylic acid to supplied ammonia issmaller than 1, excess free ammonia component contained in the lowboiling point fraction must be recovered in the below describeddistilling operation of the second step. In conducting the recovery, forexample, an ammonia absorbing column is additionally needed forrecovering ammonia. When the molar ratio of supplied carboxylic acid tosupplied ammonia is close to 1, the apparatus to be introduced can besimple.

The method of supplying the raw materials to be fed into the reaction isnot particularly limited. Like the carboxylic acid and ammonia in FIG.1, each component may be separately directly fed into the reactionvessel. Alternatively, individual components may be mixed together andreacted in advance in a separate mixing tank (first reaction vessel) tothereby prepare a liquid mixture composed mainly of an ammoniumcarboxylate, followed by supply of the liquid mixture containing theammonium carboxylate and water to a second reaction vessel in which thedehydration reaction is advanced in the presence of water. When the rawmaterials are directly fed into the reaction vessel (1) as shown in FIG.1, in the reaction vessel (1), the ammonium salt of the carboxylic acidis immediately formed from the carboxylic acid and ammonia and theammonium salt is dehydrated to thereby form the carboxylic acid amide.

Although the pressure under which the above dehydration reaction isconducted is not particularly limited, the above-mentioned pressure (2to 20 kgf/cm²) is generally employed. The reason is as follows. In thedehydration reaction, the operation is desirably conducted at hightemperatures for efficiently advancing the dehydration reaction in thepresence of supplied water or incidentally formed water without removingthem as far as possible. Thus, the reaction is generally conducted underthe above pressure. Although the pressure depends on the molar ratios ofsupplied carboxylic acid, ammonia and ammonium salt of the carboxylicacid used as raw materials, the operating temperature, etc., the aboverange of pressure is generally employed.

The reactor for use in the first step is not particularly limited aslong as the reaction vessel thereof can resist the maximum of thepressure required for carrying out the above dehydration reaction, andthere are no strict conditions on the structural form thereof.

[Second Step]

In the second step [II], the reaction mixture obtained in the abovefirst step, containing the saturated aliphatic carboxylic acid amide andwater, is distilled in a first rectifying tower (2) so that the reactionmixture is separated into low boiling point components containing thesaturated aliphatic ammonium carboxylate and water and high boilingpoint components containing the saturated aliphatic carboxylic acidamide, thereby obtaining the saturated aliphatic carboxylic acid amide.

In the first rectifying tower (2) shown in FIG. 1, the mixturecontaining carboxylic acid amide obtained in the first step is suppliedto the middle of the rectifying tower, and the highly purifiedcarboxylic acid amide and low boiling point components are recoveredfrom the bottom and the top of the rectifying tower, respectively.

The common distillation separation method is employed in the secondstep. The inside of the distilling apparatus may be held at any ofreduced pressure, atmospheric pressure and superatmospheric pressure. Ofthese, reduced pressure is preferred from the viewpoint that thedistillation temperature is lowered to thereby prevent the formation inlarge amount of by-products such as carboxylic acid amide dimerizationproducts (dicarboxylic acid amides) and carboxylic acids and nitrilecompounds both formed by pyrolysis of the dicarboxylic acid amide.

The rectification of the carboxylic acid amide in the second step isgenerally conducted in reduced pressure, preferably, a pressure of 1 to400 Torr, still preferably, 10 to 100 Torr. The temperature distributionwithin the rectifying tower is determined by the set pressure (operatingpressure), and the distillation operation is preferably conducted sothat the temperature of the bottom of the rectifying tower becomes theboiling point of carboxylic acid amide at the operating pressure.

The composition of the carboxylic acid amide containing mixture obtainedin the first step and supplied to the rectifying tower of the secondstep is mainly determined by the mixing ratio of raw materials fed inthe first step and by the reaction conditions. Although theconcentration of carboxylic acid amide is not particularly limited inthe present invention, the use of a mixture of high carboxylic acidamide concentration is generally preferred from the viewpoint that thepurity of the produced carboxylic acid amide is high and that theprocessing capacity is large irrespective of the same apparatus.Generally, the reaction mixture of 40 to 70% by weight in carboxylicacid amide concentration is supplied from the first step to the secondstep. Contained in the reaction mixture are not only the carboxylic acidamide but also water, further, ammonium carboxylates, carboxylic acidamide dimerization products (dicarboxylic acid amides) and amidinecompounds (amidine carboxylic acid salts), and still further, dependingon the mixing ratio of raw material fed in the first step, either ofcarboxylic acids and ammonia.

When an ammonia containing mixture is used as a raw material in therectification of the second step, it is difficult to completely trapammonia by means of a vapor condenser and noncondensed ammonia gas isgenerated. An ammonia absorbing step (not shown) is preferablyimplemented for absorbing this gas. Although the construction and modeof the ammonia gas absorbing step are not particularly limited, foreffectively carrying out the present invention, it is preferred that acarboxylic acid like that being a raw material for producing thecarboxylic acid amide, for example, acetic acid be used as an ammoniaabsorbing agent or ammonia trapping agent.

For example, a simple tower is used in the ammonia absorbing step. Thecarboxylic acid as an ammonia trapping agent is fed from the top of thetower while noncondensed gas composed mainly of ammonia is fed from alower part of the tower so that a gas-liquid contact is made to therebyhave ammonia, etc. absorbed. The thus recovered ammonia may betransferred to the first or third step for reutilization as a rawmaterial. This operation enables reducing the amount of raw materialrequired for obtaining a unit quantity of carboxylic acid amide (unit)to thereby attain a unit enhancement. Further, if the operation isconducted by vacuum system, the flow of ammonia gas into a vacuum pumpis prevented.

Alternatively, the generation of noncondensed ammonia gas can besuppressed by using an effluent enriched with a carboxylic acid as areflux fluid to the rectifying tower so that the carboxylic acidconcentration of the vapor is increased.

There is no particular limitation with-respect to the position of therectifying tower of the second step at which the mixture of the firststep is to be supplied. However, it is requisite that a rectificationzone exhibiting such a separation performance that the carboxylic acidamide can be satisfactorily separated from the effluent so as to avoidthe flow of the carboxylic acid amide out of the top of the rectifyingtower be disposed between the supply position and the top of therectifying tower. It is also requisite that a rectification zoneexhibiting such a separation performance that the carboxylic acid amidecan be satisfactorily separated from low boiling point components suchas carboxylic acids and ammonium salts thereof so as to avoid the flowof such low boiling point components in amounts exceeding tolerancelevels out of the bottom of the rectifying tower be disposed between thesupply position and the bottom of the rectifying tower.

The distilling apparatus used in the second step is not particularlylimited, and no particular conditions are imposed on the structural formthereof. However, generally, use is made of distilling apparatus havinga theoretical plate number of 1 to 100, preferably, 5 to 60 in therectifying tower zone.

The rectifying tower can have any arbitrary structure. For example, usecan be made of plate columns such as those including bubble cap trays,uniflux trays, flexi-trays, Nutter flow trays, ballast trays, porousplate trays, cascade trays, benturi trays, Kittel trays, recyclingtrays, chimney trays, jet trays, turbogrid trays, ripple trays, dualflow trays and baffle trays.

Further, use can be made of packed columns such as those including ringpacking, saddle packing, spray pack, Pana pack, Goodroy packing, Stedmanpacking, McMahon packing, Sulzer packing, helix and vertical platepacking.

Although the reflux ratio of the second step is not particularly limitedas long as it is set so that the outflow of carboxylic acid amide fromthe top of the rectifying tower is suppressed to the minimum extentpossible and that the effluent from the top of the rectifying tower is amixture composed mainly of ammonium salt and water, the reflux ratio isgenerally desired to range from 0.2 to 10.

The main components of the mixture recovered from the top of therectifying tower are ammonium salt of the carboxylic acid which is usedas a raw material in the reaction of the first step and water. Therecovered mixture is fed to the third step.

The component withdrawn from the bottom of the rectifying tower is ahighly purified carboxylic acid amide having 95 to 99.9% by weight ofthe carboxylic acid amide. Although this carboxylic acid amide as it ismay be used as a product, the carboxylic acid amide after separation maybe further purified to obtain a carboxylic acid amide of increasedpurity by any of the common purification methods such as there-distillation purification method, extraction distillationpurification method, crystallization purification method using varioustypes of crystallizing devices, adsorption separation method andmembrane separation method.

When the re-distillation purification method is executed, use is made ofan apparatus in which a rectification zone exhibiting such a separationperformance that the carboxylic acid amide can be satisfactorilyseparated from the high boiling point components so as to prevent highboiling point components from flowing out of the apparatus in amountsexceeding tolerance levels is disposed between the supply position andthe top of the rectifying tower. This operation can also be performed bythe simple distillation procedure, depending on the concentration ofhigh boiling point components in the purification material and theconcentration of high boiling point components tolerated by the desiredcarboxylic acid amide. Moreover, the highly purified carboxylic acidamide can be obtained by simultaneously separating low boiling pointcomponents and high boiling point components. In this operation, use ismade of a purification apparatus equipped with a rectifying tower. Lowboiling point components and high boiling point components are removedfrom an upper part and a lower part of the rectifying tower,respectively, and, simultaneously, the carboxylic acid amide isrecovered from the middle or near a lower part of the rectifying tower.Thus, the desired component is obtained. In the rectifying tower of thepurification apparatus, it is requisite that a rectification zoneexhibiting such a separation performance that the carboxylic acid amidecan be satisfactorily separated from the effluent so as to avoid theflow of the carboxylic acid amide out of the top of the rectifying towerbe disposed between the supply position and the top of the rectifyingtower. It is also requisite that a rectification zone exhibiting such aseparation performance that the carboxylic acid amide can besatisfactorily separated from low boiling point components such ascarboxylic acids and ammonium salts thereof so as to prevent such lowboiling point components from being contained in the carboxylic acidamide in amounts exceeding tolerance levels be disposed between thesupply position and the carboxylic acid amide recovering position. It isfurther requisite that a rectification zone exhibiting such a separationperformance that the carboxylic acid amide can be satisfactorilyseparated from high boiling point components such as amidine compoundsso as to prevent such high boiling point components from being containedin the carboxylic acid amide in amounts exceeding tolerance levels bedisposed between the carboxylic acid amide recovering position and thebottom of the rectifying tower. The liquid residue resulting fromexecution of these purification operations can be recycled to the firststep or third step, so that any conspicuous unit drop would not occur.

The method of fractionating the highly purified carboxylic acid amidefrom the above bottoms containing the carboxylic acid amide in highconcentration will be described in detail below.

In the present invention, preferably, an alcohol of the formula: R²OH(wherein R² represents an alkyl group having 1 to 4 carbon atoms) isadded to the high boiling point components obtained in the second step,containing the saturated aliphatic carboxylic acid amide in highconcentration, to thereby carry out an alcoholysis of by-productscontained in the high boiling point components, so that the saturatedaliphatic carboxylic acid amide is obtained. Alternatively, preferably,an alcohol of the formula: R²OH (wherein R² represents an alkyl grouphaving 1 to 4 carbon atoms) is added to a distillate obtained by furtherrectifying the high boiling point components obtained in the secondstep, containing the saturated aliphatic carboxylic acid amide in highconcentration, so as to separate the saturated aliphatic carboxylic acidamide, thereby carrying out an alcoholysis of by-products contained inthe distillate with the result that the saturated aliphatic carboxylicacid amide is obtained.

Illustratively, in the present invention, alcohol (b) is first added toa crude carboxylic acid amide containing dicarboxylic acid amide (a) tothereby prepare an alcohol solution of carboxylic acid amide. Thissolution is heated to thereby carry out such an alcoholysis ofdicarboxylic acid amide (a) that the dicarboxylic acid amide (a) isdecomposed into carboxylic acid amide (c) and carboxylic acid ester (d),as shown in the following formula. Thus, practical removal of thedicarboxylic acid amide can be attained.

wherein each of R¹ and R² independently represents an alkyl group having1 to 4 carbon atoms.

This alcoholysis enables obtaining an alcohol solution of highlypurified carboxylic acid amide (c) which does not contain dicarboxylicacid amides. Further purified carboxylic acid amide can be obtained byseparating an alcohol and the carboxylic acid ester from this alcoholsolution of highly purified carboxylic acid amide, according tonecessity. The carboxylic acid ester (d) can be separated simultaneouslywith the alcoholysis or after the completion of the alcoholysis.

Lower alcohols are preferably used as the alcohol (b), and anappropriate one is selected from thereamong, taking into account theeasiness in separating the obtained carboxylic acid amide (c) from thecarboxylic acid ester (d) and in the purification thereof. Preferably,use is made of an alcohol represented by the formula R²OH (wherein R²represents an alkyl group having 1 to 4 carbon atoms), which is, forexample, methanol, ethanol, n-propanol, isopropanol or n-butanol. Ofthese alcohols, methanol is especially preferred.

The above alcoholysis reaction is carried out at 50 to 200° C.,preferably, 80 to 170° C. Conducting the alcoholysis reaction attemperatures lower than the above range retards the rate of alcoholysisof the dicarboxylic acid amide to thereby prolong the reaction time. Onthe other hand, raising the reaction temperature over the above rangecauses a thermal deterioration of the carboxylic acid amide to therebylower the yield of carboxylic acid amide although the reaction rate isincreased.

The reaction time is varied depending on the reaction temperature, themixing molar ratio of dicarboxylic acid amide to alcohol, theconcentration of dicarboxylic acid amide at the initiation of reaction,the mode of reaction, etc.

The reaction time may be set in conformity with conditions of these andthe reaction time is not particularly limited. However, although theoptimum reaction time is varied depending on the reaction temperatureand mixing molar ratio of dicarboxylic acid amide to alcohol, thethermal deterioration of the carboxylic acid amide is generallyproportional to the reaction time, so that it is preferred that thereaction be carried out within a short period of time.

The inside of the reactor may be held at any of reduced pressure,atmospheric pressure and superatmospheric pressure. Of these,atmospheric pressure is preferred. The reaction in reduced pressurecauses a lowering of reaction performance and a prolongation of reactiontime. The reaction under superatmospheric pressure requires a pressureresistant apparatus, which is disadvantageous from the viewpoint ofcost.

Although the amount of alcohol added to-the carboxylic acid amide is notparticularly limited, it is preferred that the amount be determined inconformity with the amount of dicarboxylic acid amide contained in thecarboxylic acid amide. Generally, the alcohol is preferably added insuch an amount that the molar ratio of dicarboxylic acid amide toalcohol ranges from 1/1 to 1/1000, especially, from 1/10 to 1/100. Anincrease of the alcohol concentration lowers the boiling point of thesystem. Therefore, when it is intended to carry out the reaction atatmospheric pressure, the reaction temperature becomes so low that thereaction cannot be effectively carried out. The reaction temperature canbe raised and the reaction time shortened by using a closed reactor andby carrying out the reaction under super-atmospheric pressure. However,this reaction requires a high pressure vessel. When the alcoholconcentration is low, the alcoholysis (methanolysis) of dicarboxylicacid amide has a low reaction rate, also leading to disadvantageousconditions.

The reaction mode for carrying out the above alcoholysis reaction is notparticularly limited. The reaction may be carried out under refluxconditions by setting the reaction temperature at the boiling point ofthe reaction fluid or may be operated with the vaporization suppressedat temperatures lower than the boiling point. The reaction may also becarried out at super-atmospheric pressure by using a closed reactionvessel.

Moreover, the employable reactor is not particularly limited, and thereis no particular limitation on its structural form. When the reaction iscarried out under reflux conditions, it is desirable to use a reactionvessel having a vapor condenser directly connected therewith. On theother hand, when the reaction is carried out at super-atmosphericpressure, it is desirable to use a high pressure reaction vessel whichcan resist the operating pressure.

The carboxylic acid amide is unstable in water and slowly decomposes byabsorbing moisture from the air. Therefore, the reaction vessel andauxiliary facilities such as raw material tank and product tank arepreferably held in atmosphere of, for example, nitrogen or dry air.

The separation of the carboxylic acid ester formed by the abovealcoholysis can be performed simultaneously with the alcoholysis orafter the completion of the alcoholysis. For example, low boiling pointvapor containing the carboxylic acid ester may be generated byconducting the alcoholysis reaction at the boiling point of the reactionfluid. The boiling point of formed carboxylic acid ester is considerablylower than that of the carboxylic acid amide, so that the carboxylicacid ester is contained in the vapor in higher concentration. Thecarboxylic acid ester can be removed by expelling a condensate of thevapor outside the system. The separation of the carboxylic acid estercan be efficiently conducted by the use of a rectifying tower.

In the present invention, preferably, water is added to a residue(liquid residue) obtained by further rectifying the high boiling pointcomponents containing the saturated aliphatic carboxylic acid amide inhigh concentration according to any of the above various methods so asto separate the saturated aliphatic carboxylic acid amide, therebycarrying out a hydrolysis of side reaction products contained in theresidue with the result that a mixture containing the saturatedaliphatic carboxylic acid amide and saturated aliphatic ammoniumcarboxylate is obtained, this mixture being recycled to the first orsecond step.

This process will further be illustrated below.

In the production of the carboxylic acid amide through the dehydrationreaction of the ammonium carboxylate, main reaction by-products are theabove dicarboxylic acid amide represented by the general formula(R¹CO)₂NH (wherein R¹ is as defined above) and carboxylic acid salt ofamidine represented by the formula R¹C(NH)(NH₂) (wherein R¹ is asdefined above). These reaction by-products are generally contained in aconcentrated state in the residue (liquid residue) resulting from aseparation of the carboxylic acid amide from the synthetic reactionfluid by the distillation, crystallization or other method.

In the present invention, the above hydrolytic reaction by adding waterto the residue converts the dicarboxylic acid amide to correspondingcarboxylic acid amide, carboxylic acid and the like and converts theamidine carboxylic acid salt to corresponding carboxylic acid amide,ammonium carboxylate and the like, so that a mixture composed mainly ofthe carboxylic acid amide and ammonium carboxylate (including watersupplied for hydrolysis) can be obtained.

The thus obtained mixture composed mainly of the carboxylic acid amideand ammonium carboxylate can be recycled to the first or second step foruse as a part of raw material in the production of the carboxylic acidamide from the ammonium carboxylate or for reutilization (recycle) inthe rectifying tower of the second step. Thus, high efficiency isensured.

When a selective hydrolysis of side reaction products is conducted byadding water to the residue or by adding the residue to water, it ispreferred that conditions under which a hydrolysis of the carboxylicacid amide as a principal component can be suppressed to the minimumextent possible be adopted. It is believed that the dicarboxylic acidamide and amidine carboxylic acid salt are more readily hydrolyzed thanthe carboxylic acid amide. Although the presence of an acid or basecatalyst can increase the rate of hydrolysis of reaction by-products,the carboxylic acid amide also becomes readily hydrolyzed, therebyunfavorably lowering the selectivity.

The amount of water added to the residue is generally adjusted so thatthe water accounts for 1 to 50% by weight based on the weight of thewhole of the residue plus added water, although depending on the amountof reaction by-products contained in the residue. When the amount ofwater exceeds the above range, the hydrolysis of the carboxylic acidamide is advanced so that the selectivity is lowered. On the other hand,when it is smaller than the above range, the hydrolysis of reactionby-products is retarded to thereby prolong the reaction time to anextent of inefficiency.

This hydrolysis reaction is generally conducted at 50 to 300° C.,preferably, 100 to 200° C. at atmospheric pressure. This hydrolysisreaction may be conducted under super-atmospheric pressure. The residuehaving been subjected to the selective hydrolysis reaction of sidereaction products may be individually subjected to a separation of thecarboxylic acid amide or may be first mixed with the synthetic reactionfluid of carboxylic acid amide and then subjected to a separation of thecarboxylic acid amide.

The above process in which water is added to the residue (liquidresidue) resulting from fractionation of the saturated aliphaticcarboxylic acid amide to thereby carry out a hydrolysis of side reactionproducts contained in the residue with the result that a mixturecontaining the saturated aliphatic carboxylic acid amide and saturatedaliphatic ammonium carboxylate is obtained can be effectively applied tonot only the above residue resulting from fractionation of the highlypurified saturated aliphatic carboxylic acid amide, obtained in thesecond step of the present invention, but also the residue resultingfrom fractionation of the saturated aliphatic carboxylic acid amide,obtained by the conventional method.

[Third Step]

In the third step [III] shown in FIG. 1, part of water is distilled offfrom the low boiling point components obtained in the second step in asecond rectifying tower (3) to thereby obtain an aqueous solution ofsaturated aliphatic ammonium carboxylate, and this aqueous solution isrecycled to the reactor (1) of the first step.

Illustratively, this third step comprises distilling off part of waterfrom the low boiling point components (mixture) recovered from the topof the first rectifying tower in the second step, containing theammonium carboxylate and water as principal components, to therebyobtain an aqueous solution of ammonium carboxylate as a principalcomponent and feeding the obtained aqueous solution of ammoniumcarboxylate (recovered component) to the first step.

The rectifying operation is performed according to the same commondistillation separation method as in the second step. The inside of thedistilling apparatus may be held at any of reduced pressure, atmosphericpressure and superatmospheric pressure. Of these, reduced pressure ispreferred. Although the distillation temperature is not particularlylimited, distillation at high temperatures accelerates the decompositionreaction of ammonium carboxylates to thereby invite the flow of ammoniatogether with water from an upper part of the distilling apparatus.Thus, an aqueous ammonia separating step for distilling ammonia from thefraction is additionally needed. Therefore, it is generally preferredthat the distillation be conducted at 150° C. or below, especially, 120°C. or below as measured at the lowest part inside the apparatus(distillation column bottom temperature).

It is preferred that the rectifying operation be performed in reducedpressure, especially, reduced pressure of 1 to 600 Torr, stillespecially, 10 to 400 Torr and yet still especially 50 to 300 Torr. Thetemperature distribution within the rectifying tower is determined bythe operating pressure.

The composition of the fraction recovered from the top of the firstrectifying tower of the second step and supplied as a charge material tothe third step is mainly determined by the mixing ratio of raw materialsfed in the first step and by the reaction conditions.

The fraction recovered from the top of the first rectifying tower isgenerally a liquid mixture containing not only ammonium carboxylate andwater but also carboxylic acids or ammonia.

Although this liquid mixture (charge material for rectification in thethird step) may be directly fed into the distilling apparatus (3) tothereby conduct the distilling operation, it is preferred that acarboxylic acid or ammonia be fed into the liquid mixture to therebyattain such a adjustment that the carboxylic acid is present slightly inexcess in the liquid mixture to be supplied to the third step with noneof free ammonia present therein, prior to conducting the distillingoperation of the third step. That is, it is preferred that such anadjustment that the molar ratio of ammonium carboxylate to carboxylicacid in the liquid mixture ranges from 1/0.1 to 1/10, especially, from1/0.25 to 1/2 be made prior to supplying the liquid mixture to the thirdstep.

When the carboxylic acid is added in excess of the ammonium saltthereof, the decomposition reaction of the ammonium salt is suppressed.Further, even if ammonia gas is generated by the decomposition reactionof the ammonium carboxylate, the carboxylic acid exerts an action astrapping agent, so that the flow of ammonia from an upper part of thedistilling apparatus (3) of the third step can be suppressed. Moreover,the water content of the ammonium carboxylate component recovered from alower part of this apparatus and recycled to the first step can bereduced and the theoretical plate number of the distillation column usedin the third step can be decreased thereby.

However, when the carboxylic acid is added to the purification chargematerial of the third step in excess of the above ratio, the carboxylicacid is distilled out from an upper part of the apparatus. Thus, a stepfor separating the carboxylic acid and water is additionally needed,which is disadvantageous from the viewpoint of cost.

The distilling apparatus used in the third step is not particularlylimited, and no-particular conditions are imposed on the structural formthereof. However, generally, use is made of distilling apparatus havinga theoretical plate number of 1 to 100, preferably, 5 to 60 in therectifying tower zone. The rectifying tower can have any arbitrarystructure. For example, use can be made of the above-mentioned platecolumns and packed-columns.

Although the charge material supply position is not particularlylimited, it is desired that a rectification zone exhibiting such aseparation performance that the carboxylic acid or ammonia is notdistilled out of the top of the rectifying tower in an amount exceedingtolerance level be disposed between the charge material supply positionand the top of the rectifying tower and that a rectification zoneexhibiting such a separation performance that water is not dischargedfrom the bottom of the rectifying tower in an amount exceeding tolerancelevel be disposed between the charge material supply position and thebottom of the rectifying tower.

The rectification conditions are preferably so set that the flow ofammonium carboxylate, carboxylic acid and ammonia from the top of therectifying tower can be suppressed and that, as completely as possible,only water is distilled off the tower top. It is generally desired thatthe reflux ratio range from 0.2 to 10.

The recovered material from the tower bottom is a mixture comprisingammonium carboxylate and water as main component, and optionallycarboxylic acid.

The mixture is fed to the first step to reutilize thereof.

Ammonia may be added in an amount equimolar to that of the carboxylicacid contained in the mixture recovered from the tower bottom (recoveredmaterial) so that the carboxylic acid reacts with ammonia to formammonium carboxylate, thereby increasing the purity of ammoniumcarboxylate.

As apparent from the foregoing description, the process for producing acarboxylic acid amide according to the present invention ischaracterized in that the dehydration reaction of the ammoniumcarboxylate obtained from the carboxylic acid and ammonia is advanced inthe presence of a large amount of water which is contained in thecomponents recycled from the second step and/or third step and formed inaccordance with the progress of the dehydration reaction. Further, it ischaracterized in that the dehydration reaction is advanced under suchconditions that the undesired formation of carboxylic acid amidedimerization products, nitrile compounds and amidine compounds issuppressed. As a result, production of the carboxylic acid amide with ahigh selectivity has been realized.

In the prior art method, water incidentally formed during the productionof the carboxylic acid amide is recovered together with the carboxylicacid or ammonia. By contrast, in the present invention, water isfractionated as a liquid mixture composed mainly of water and theammonium carboxylate; separation of part of water from this liquidmixture is performed by carrying out distilling operation at relativelylow temperatures such that the rate of decomposition of ammoniumcarboxylate is low; and the separation is conducted in the presence ofthe carboxylic acid with the result that efficient removal of water withthe use of simple apparatus has been realized.

Moreover, the dicarboxylic acid amide incidentally produced during theproduction of the carboxylic acid amide has a boiling point which isclose to that of the carboxylic acid amide and the thermal stability ofthe carboxylic acid amide is poor around the boiling point of thedicarboxylic acid amide. Thus, in the prior art, it has been difficultto separate them by a simple method, as mentioned hereinbefore. However,in the present invention, the alcohol is added to the crude carboxylicacid amide containing the dicarboxylic acid amide and heated to therebyperform the alcoholysis of the dicarboxylic acid amide into thecarboxylic acid amide and carboxylic acid ester. Thus, substantiallycomplete removal of the dicarboxylic acid amide while preventingdeterioration of the carboxylic acid amide is attained by a simplemethod such as distillation utilizing the significant boiling pointdifference between the carboxylic acid amide and the carboxylic acidester. Consequently, obtaining the highly purified carboxylic acid amidehas been enabled.

EFFECT OF THE INVENTION

In the process for producing a carboxylic acid amide according to thepresent invention, the dehydration. reaction of the ammonium carboxylateobtained from the carboxylic acid and ammonia is advanced under a supplyof water to thereby suppress the formation of by-products such ascarboxylic acid amide dimerization products, nitrile compounds andamidine compounds to an extremely low level. As a result, production ofthe carboxylic acid amide with a high selectivity has been realized.

In the third step thereof, the separation of water from the ammoniumcarboxylate is performed in the presence of free carboxylic acid tothereby efficiently remove water by means of simple apparatus with theresult that fractionation of the ammonium carboxylate has been realized.

Therefore, the present invention has enabled simplification of theproduction steps, has ensured excellent operation efficiency, haslowered production facility constructing and operating costs, hasreduced the amount of by-products and has enabled producing the highlypurified carboxylic acid amide at a high selectivity, at a high yieldand on an industrial scale.

Further, the present invention has established a process for producingthe carboxylic acid amide, which can attain an effective utilization ofthe residue resulting from recovery of the carboxylic acid amide andavoids accumulation, in the reaction system, of side reaction productsthat are detrimental to the production of the carboxylic acid amide.

Still further, the present invention has established a process forpurifying the carboxylic acid amide, in which the crude carboxylic acidamide containing the dicarboxylic acid amide is purified to therebyenable producing the highly purified carboxylic acid amide which issuitably usable as a raw material in the synthesis of an N-vinylcarboxylic acid amide, simply, at a high yield and with an industrialadvantage.

EXAMPLE

The present invention will now be illustrated in greater detail withreference to the following Examples, which in no way limit the scope ofthe invention.

Example A1

[First Step]

Acetic acid, ammonia and the 20:15:10 (in molar ratio) mixture ofammonium acetate, acetic acid and water recycled from the third stepwere continuously supplied in respective rates of 61.2, 24.5 and 73.3kg/hr to a reaction vessel of 1 m³ in volume equipped with an agitator,which reaction vessel had its temperature and pressure maintained at170° C. and 5 kgf/cm², respectively. Simultaneously, a mixture (liquid)containing acetamide in a concentration of 54% by weight wascontinuously withdrawn at a rate of 159 kg/hr.

The amount of steady-state water in the reaction vessel was 47 mol per100 mol of the total of acetic acid, ammonia, ammonium acetate,acetamide and water. In the withdrawn mixture, the ammonium acetateconversion was 72% and the acetamide selectivity was 99%.

[Second Step]

The mixture (liquid) containing acetamide in a concentration of 54% byweight, withdrawn from the first step, was continuously supplied to themiddle of a rectifying tower of 22 in theoretical plate number packedwith McMahon packing at a rate of 159 kg/hr. The distilling operationwas conducted at a pressure of 50 Torr and at a reflux ratio of 2. Thetemperature of the bottom of the distillation column was 142° C.

Concentrate was recovered from the tower bottom at a rate of 85.0 kg/hr.The acetamide concentration of the concentrate was 99% by weight. Amixture of ammonium acetate and water was recovered from the tower topat a rate of 74.1 kg/hr.

[Third Step]

Acetic acid was added to the mixture of ammonium acetate and water beingthe low boiling point components of the second step, obtained as therectifying tower overhead, so that the molar ratio of ammonium acetateto acetic acid was 1:0.75. The resultant mixture was continuouslysupplied to a lower plate of a rectifying tower of 10 in theoreticalplate number packed with McMahon packing at a rate of 99.3 kg/hr. Thedistilling operation was conducted at a pressure of 200 Torr and at areflux ratio of 2. The temperature of the bottom of the distillationcolumn was 100° C.

Consequently, a 1:0.75:0.5 (in molar ratio) mixture of ammonium acetate,acetic acid and water having an ammonium acetate concentration of 59% byweight was recovered from the tower bottom at a rate of 73.4 kg/hr andan overhead of 99.5% by weight in water concentration was recovered fromthe tower top at a rate of 25.9 kg/hr. The bottoms were reutilized asthe raw material in the reaction of the first step.

Comparative Example A1

1000 kg of ammonium acetate was charged into a reaction vessel of 2 m³in internal volume equipped with an agitator and also equipped with arectifying tower of 14 in theoretical plate number packed with McMahonpacking and slowly heated. The reaction was carried out while recoveringwater formed in accordance with the advance of dehydration reaction froman upper part of the apparatus. The internal temperature of the reactionvessel was raised to, finally, about 180° C. The acetamide concentrationof recovered reaction fluid was about 60% by weight, and the ammoniumacetate conversion and acetamide selectivity were 95% and 94%,respectively.

Example B1

[Method of Synthesizing Acetamide]

Acetic acid and ammonia were continuously supplied in respective ratesof 60 and 17 kg/hr to a reaction vessel of 0.6 m³ in volume equippedwith an agitator, which reaction vessel had its temperature and pressuremaintained at 170° C. and 6 kgf/cm², respectively. Simultaneously, areaction mixture (liquid) containing acetamide in a concentration ofabout 54% by weight was continuously withdrawn from the reaction vesselat a rate of 77 kg/hr. The reaction was carried out at a residence timeof 6 hr, and the ammonium acetate conversion and acetamide selectivitywere 72% and 99%, respectively.

The composition of the reaction mixture is given in Table B1.

TABLE B1 Composition wt. % acetamide 53.7 water 18.7 ammonium acetate27.1 acetic acid salt of acetamidine  0.4

[Method of Separating Acetamide]

The reaction mixture (liquid) containing acetamide in a concentration ofabout 54% by weight was continuously supplied to the middle of arectifying tower of 22 in theoretical plate number packed with McMahonpacking at a rate of 77 kg/hr. The distilling operation was conducted ata pressure of 50 mmHg and at a reflux ratio of 2. The temperature of thebottom of the distillation column was 143° C.

Concentrate was recovered from the tower bottom at a rate of 42 kg/hr.The acetamide concentration of the concentrate was 99% by weight. Amixture of ammonium acetate and water was recovered from the tower topat a rate of 35 kg/hr.

The resultant concentrate was subjected to a simple distillationperformed at 110° C. and at a pressure of 10 mmHg while continuouslysupplying the same at a rate of 42 kg/hr to thereby separate acetamidecomponent. Thus, highly purified acetamide (99.7% by weight in purity)was obtained. The residue was recovered from a lower part of theapparatus at a rate of 4 kg/hr. The composition of the residue is asspecified in the following Table B2.

TABLE B2 Composition wt. % acetamide 97.7  diacetamide 0.3 acetic acidsalt of acetamidine 2.0

[Method of Hydrolyzing Residue]

The residue with the composition of Table B2 and water were continuouslyadded to a pressure resistant reaction vessel at rates of 3 and 1 kg/hr,respectively and reacted at 170° C., an operating pressure of 5.5kgf/cm² and a residence time of 6 hr. Simultaneously, the reaction fluidwas withdrawn therefrom at a rate of 4 kg/hr.

The composition of recovered reaction product is specified in thefollowing Table B3. After the completion of the reaction, thediacetamide conversion and conversion of acetic acid salt of acetamidinewere 95 and 80%, respectively.

TABLE B3 Composition wt. % acetamide 59.6 diacetamide  0.01 acetic acidsalt of acetamidine  0.3 water 20.7 ammonium acetate 19.4

The obtained reaction fluid was recycled to the acetamide purifyingstage (second step) and used as part of the feed material.

Example B2

A carboxylic acid amide was produced in the same manner as in ExampleB1, except that the reaction conditions were as follows.

Specifically, acetic acid and ammonia were continuously supplied inrespective rates of 60 and 17 kg/hr to a pressure resistant reactionvessel of 0.6 m³ in volume and, further, the residue with thecomposition of Table B2, obtained in the same manner as in Example B1,was continuously fed thereto at a rate of 3 kg/hr. The reaction fluidwas continuously withdrawn from the reaction vessel at a rate of 80kg/hr. The reaction was carried out at 170° C. and at a residence timeof 6 hr with the reaction pressure set at 5.5 kgf/cm².

Thereafter, the production of the carboxylic acid amide was performed inthe same manner as in Example B1.

The composition of obtained reaction product is specified in thefollowing Table B4.

TABLE B4 Composition wt. % acetamide 60.7 diacetamide  0.01 acetic acidsalt of acetamidine  0.4 water 18.3 ammonium acetate 20.6

Comparative Example B1

A carboxylic acid amide was produced in the same manner as in ExampleB2, except that the reaction conditions were as follows.

Specifically, acetic acid and ammonia were continuously fed inrespective rates of 60 and 17 kg/hr to the same pressure resistantreaction vessel as used in Example B2. The reaction fluid wascontinuously withdrawn from the reaction vessel at a rate of 77 kg/hr.The reaction was carried out at 170° C. and at a residence time of 6 hrwith the reaction pressure set at 6 kgf/cm².

Thereafter, the production of the carboxylic acid amide was performed inthe same manner as in Example B2.

The composition of obtained reaction product is specified in thefollowing Table B5.

TABLE B5 Composition wt. % acetamide 54.1 diacetamide  0.01 acetic acidsalt of acetamidine  0.4 water 18.3 ammonium acetate 27.2

The ratios of diacetamide and acetic acid salt of acetamidine toacetamide were the same as in Example B2 in which the residue was addedas part of the charge material.

Reference Example B1

[Method of Synthesizing Acetamide]

Acetic acid and ammonia were continuously supplied in respective ratesof 192 and 34 kg/hr to a reaction vessel of 2 m³ in volume equipped withan agitator and also equipped with a rectifying tower of 14 intheoretical plate number packed with McMahon packing. The operation wascarried out at a reaction vessel internal temperature of 170° C., areflux ratio of 2 and a residence time of 6 hr.

A reaction fluid of 60% by weight in acetamide concentration wasrecovered from the reaction vessel part at a rate of 186 kg/hr, and a1:0.06 (in molar ratio) mixture of water and acetic acid was recoveredfrom the top of the distillation column at a rate of 40 kg/hr. Theconversion of ammonium acetate obtained from acetic acid and ammonia was95% and the acetamide selectivity was 95%.

[Method of Separating Acetamide]

The reaction mixture (liquid) containing acetamide in a concentration of60% by weight was continuously supplied to the middle of a rectifyingtower of 22 in theoretical plate number packed with McMahon packing at arate of 186 kg/hr. The distilling operation was conducted at a pressureof 50 mmHg and at a reflux ratio of 2. The temperature of the bottom ofthe distillation column was 143° C. Concentrate was recovered from thetower bottom at a rate of 116 kg/hr. The acetamide concentration of theconcentrate was 97% by weight.

The resultant concentrate was subjected to a simple distillationperformed at 110° C. and at a pressure of 10 mmHg while continuouslysupplying the same at a rate of 116 kg/hr to thereby separate acetamidecomponent. Thus, highly purified acetamide (99.5% by weight in purity)was obtained. The residue was recovered from a lower part of theapparatus at a rate of 12 kg/hr. The composition of the residue is asspecified in the following Table B6.

TABLE B6 Composition wt. % acetamide 92.2  diacetamide 0.3 acetic acidsalt of acetamidine 7.5

[Method of Hydrolyzing Residue]

The residue with the composition of Table B6 and water were continuouslyadded to a pressure resistant reaction vessel at rates of 12 and 4kg/hr, respectively and, simultaneously, the reaction fluid waswithdrawn therefrom at a rate of 16 kg/hr. The reaction was carried outat 170° C., an operating pressure of 5.5 kgf/cm² and a residence time of6 hr. The composition of recovered reaction product is specified in thefollowing Table B7. After the completion of the reaction, thediacetamide conversion and conversion of acetic acid salt of acetamidinewere 96 and 91%, respectively.

TABLE B7 Composition wt. % acetamide 59.6 diacetamide  0.01 acetic acidsalt of acetamidine  0.5 water 20.7 ammonium acetate 19.4

The obtained reaction fluid was recycled to the acetamide synthesizingstage and used as part of the raw material for acetamide.

Example C1

A mixture comprising 99.5% by weight of acetamide and 0.3% by weight ofdiacetamide together with water, acetic acid, etc. was continuously fedat a rate of 90 kg/hr into a reaction vessel of 4 m³ in volume equippedwith a vapor condenser, and also methanol was continuously fed thereintoat a rate of 10 kg/hr. The operation was carried out at a reactionvessel temperature of 115° C. and at a reaction fluid volume of 2 m³.The reaction fluid was continuously withdrawn at a rate of 100 kg/hr.The rise of a vapor composed mainly of methanol from an upper part ofthe reaction vessel was recognized, which was condensed by means of avapor condenser and recycled to the reaction vessel. The diacetamideconversion was 92%.

Example C2

A mixture comprising 99.5% by weight of acetamide and 0.3% by weight ofdiacetamide together with water, acetic acid, etc. was continuously fedat a rate of 95 kg/hr into a reaction vessel of 4 m³ in volume, and alsomethanol was continuously fed thereinto at a rate of 5 kg/hr. Theoperation was carried out at a reaction vessel temperature of 120° C.and at a reaction fluid volume of 3 m³. The reaction fluid wascontinuously withdrawn at a rate of 100 kg/hr. The diacetamideconversion was 90%.

Example C3

Low boiling point components were distilled from the reaction fluidobtained in Example C1, thereby obtaining acetamide product (highlypurified acetamide) of at least 99.9% by weight in purity having adiacetamide concentration of less than 0.03% by weight at a rate of 89kg/hr.

Example D1

Acetic acid was added to an aqueous ammonium acetate solution comprisingammonium acetate and water in a molar ratio of 1:4 in an amount of 0.7time the mole of ammonium acetate. The resultant mixture wascontinuously supplied to a lower plate of a rectifying tower of 10 intheoretical plate number packed with McMahon packing at a rate of 100kg/hr. The distilling operation was conducted at a pressure of 200 Torrand at a reflux ratio of 2. The temperature of the bottom of thedistillation column was 100° C.

Consequently, a 1:0.7:0.5 (in molar ratio) mixture of ammonium acetate,acetic acid and water was recovered from the tower bottom at a rate of67 kg/hr and an overhead of 99.5% by weight in water concentration wasrecovered from the tower top at a rate of 33 kg/hr.

Example D2

Acetic acid was added to an aqueous ammonium acetate solution comprisingammonium acetate, acetic acid and water in a molar ratio of 1:0.2:5,thereby preparing an aqueous solution comprising ammonium acetate,acetic acid and water in a molar ratio of 1:1:5. The resultant aqueoussolution was continuously supplied to a lower plate of a rectifyingtower of 10 in theoretical plate number packed with McMahon packing at arate of 100 kg/hr. The distilling operation was conducted at a pressureof 200 Torr and at a reflux ratio of 2. The temperature of the bottom ofthe distillation column was 100° C.

Consequently, a 1:1:0.2 (in molar ratio) mixture of ammonium acetate,acetic acid and water was recovered from the tower bottom at a rate of61 kg/hr and an overhead of 98.0% by weight in water concentration wasrecovered from the tower top at a rate of 39 kg/hr.

Example D3

Acetic acid was added to an aqueous ammonium acetate solution comprisingammonium acetate and water in a molar ratio of 1:4 in an amount of 0.7time the mole of ammonium acetate. The resultant mixture wascontinuously supplied to a lower plate of a rectifying tower of 10 intheoretical plate number packed with McMahon packing at a rate of 100kg/hr. The distilling operation was conducted at a pressure of 70 Torrand at a reflux ratio of 2. The temperature of the bottom of thedistillation column was 85° C.

Consequently, a 1:0.7:0.4 (in molar ratio) mixture of ammonium acetate,acetic acid and water was recovered from the tower bottom at a rate of66 kg/hr and an overhead of 99.0% by weight in water concentration wasrecovered from the tower top at a rate of 34 kg/hr.

Comparative Example D1

An aqueous ammonium acetate solution comprising ammonium acetate andwater in a molar ratio of 1:4 was continuously supplied to a lower plateof a rectifying tower of 10 in theoretical plate number packed withMcMahon packing at a rate of 100 kg/hr. The distilling operation wasconducted at a pressure of 200 Torr and at a reflux ratio of 2. Thetemperature of the bottom of the distillation column was 100° C.

Consequently, a 1:0.7:0.5 (in molar ratio) mixture of ammonium acetate,acetic acid and water was recovered from the tower bottom at a rate of51 kg/hr and a 10% by weight aqueous ammonia was recovered from thetower top at a rate of 49 kg/hr.

Comparative Example E1

The flow of materials, products, etc. is shown by arrows in FIG. 2.

[First Step]

Acetic acid was newly continuously supplied in a rate of 114 kg/hr to areaction vessel of 2 m³ in internal volume equipped with an agitator andalso equipped with a rectifying tower of 14 in theoretical plate numberpacked with McMahon packing, from a lower part of the rectifying tower.Also, ammonia was continuously supplied at a rate of 32 kg/hr theretofrom the reaction vessel part. The recycle acetic acid and ammoniumacetate, recovered from the second and third steps, were supplied atrespective rates of 72 and 8 kg/hr together with newly added acetic acidto the reaction vessel from an upper part of the rectifying tower. Theoperation was carried out at a reaction vessel internal temperature of170° C., a reflux ratio of 2 and an internal capacity of 1.4 m³. Areaction fluid of 60% by weight in acetamide concentration was recoveredfrom the reaction vessel part at a rate of 187 kg/hr, and an aceticacid/water mixture of 88% by weight in water concentration was recoveredfrom the top of the rectifying tower at a rate of 39 kg/hr. Theconversion of ammonia supplied as a raw material was 100%, theconversion of ammonium acetate was 95%, and the acetamide selectivitywas 95%.

[Second Step]

The reaction fluid containing acetamide in a concentration of 60% byweight, recovered in the first step, was continuously supplied to themiddle of a rectifying tower of 22 in theoretical plate number packedwith McMahon packing at a rate of 187 kg/hr. The distilling operationwas conducted at a pressure of 50 Torr and at a reflux ratio of 2. Thedistillation temperature was 143° C. Acetamide concentrate was recoveredfrom the tower bottom at a rate of 113 kg/hr. The acetamideconcentration of the concentrate was 99% by weight. A 11:1:1 (in molarratio) mixture of acetic acid, ammonium acetate and water was recoveredfrom the tower top at a rate of 74 kg/hr. The recovered mixture wasdirectly fed for use as a raw material in the reaction of the firststep.

[Third Step]

The acetic acid/water mixture obtained in the first step wascontinuously supplied to a lower plate of a rectifying tower of 50 intheoretical plate number packed with McMahon packing at a rate of 39kg/hr. The distilling operation was conducted at a pressure of 70 Torrand at a reflux ratio of 2. The distillation temperature was 55° C.

As a result, an acetic acid/water mixture of 90% by weight in aceticacid concentration was recovered from the tower bottom at a rate of 5kg/hr. The recovered mixture composed mainly of acetic acid wasreutilized as a raw material in the reaction of the first step.

What is claimed is:
 1. A process for producing a saturated aliphaticcarboxylic acid amide, comprising reacting a saturated aliphaticcarboxylic acid with ammonia to thereby obtain a saturated aliphaticammonium carboxylate and subjecting the saturated aliphatic ammoniumcarboxylate to a dehydration reaction for obtaining a saturatedaliphatic carboxylic acid amide, wherein the dehydration reaction of thesaturated aliphatic ammonium carboxylate is conducted in a reactionvessel under a pressure of at least 2 kgf/cm² in a continuous manner andthe amount of water present at steady state ranges from 20 to 70 mol per100 mol of the total of the saturated aliphatic carboxylic acid, theammonia, the saturated aliphatic ammonium carboxylate, the saturatedaliphatic carboxylate acid amide, and the water.
 2. The process asclaimed in claim 1, which comprises: a first step comprising: feeding toa reaction vessel, as raw materials, (a) the saturated aliphaticcarboxylic acid, ammonia and water, and/or (b) the saturated aliphaticammonium carboxylate and water, and conducting the dehydration reactionof the saturated aliphatic ammonium carboxylate in the presence of waterto thereby form the saturated aliphatic carboxylic acid amide and water;a second step comprising distilling a reaction mixture obtained in thefirst step containing the saturated aliphatic carboxylic acid amide andwater, in a first rectifying tower so that the reaction mixture isseparated into low boiling point components containing the saturatedaliphatic ammonium carboxylate and water and high boiling pointcomponents containing the saturated aliphatic carboxylic acid amide,thereby obtaining the saturated aliphatic carboxylic acid amide; and athird step comprising distilling off part of water from the low boilingpoint components obtained in the second step in a second rectifyingtower to thereby obtain an aqueous solution of saturated aliphaticammonium carboxylate and feeding the aqueous solution to the first step.3. The process as claimed in claim 1, wherein the dehydration reactionof the saturated aliphatic ammonium carboxylate is conducted at 130 to250° C. under a pressure of 2 to 20 kgf/cm².
 4. The process as claimedin claim 2, wherein the high boiling point components obtained in thesecond step, containing the saturated aliphatic carboxylic acid amide inhigh concentration, are further rectified to thereby separate thesaturated aliphatic carboxylic acid amide from the high boiling pointcomponents to obtain a residue and wherein water is added to the residueto thereby hydrolyze side reaction products contained in the residue sothat a mixture containing the saturated aliphatic carboxylic acid amideand the saturated aliphatic ammonium carboxylate is obtained, saidmixture recycled to the first step or the second step.
 5. The process asclaimed in claim 2, wherein an alcohol of the formula: R²OH (wherein R²represents an alkyl group having 1 to 4 carbon atoms) is added to thehigh boiling point components obtained in the second step, containingthe saturated aliphatic carboxylic acid amide in high concentration, tothereby carry out an alcoholysis of by-products contained in the highboiling point components, so that the saturated aliphatic carboxylicacid amide is obtained.
 6. The process as claimed in claim 2, wherein analcohol of the formula: R²OH (wherein R² represents an alkyl grouphaving 1 to 4 carbon atoms) is added to a distillate obtained by furtherrectifying the high boiling point components obtained in the secondstep, containing the saturated aliphatic carboxylic acid amide in highconcentration, so as to separate the saturated aliphatic carboxylic acidamide, thereby carrying out an alcoholysis of by-products contained inthe distillate with the result that the saturated aliphatic carboxylicacid amide is obtained.
 7. The process as claimed in claim 5, whereinthe alcohol is methanol and the by-product is a saturated aliphaticdicarboxylic acid amide.
 8. The process as claimed in claim 6, whereinthe alcohol is methanol and the by-product is a saturated aliphaticdicarboxylic acid amide.
 9. The process as claimed in claim 2, wherein,in the third step, a saturated aliphatic carboxylic acid is added to thelow boiling point components obtained in the second step, containing thesaturated aliphatic ammonium carboxylate and water, and distilled in thepresence of 0.1 to 10 mol of the saturated aliphatic carboxylic acid permol of the saturated aliphatic ammonium carboxylate to thereby separatewater.
 10. The process as claimed in any of claim 1 to 9, wherein thesaturated aliphatic carboxylic acid is represented by the formula:R¹COOH (wherein R¹ represents an alkyl group having 1 to 4 carbonatoms), the saturated aliphatic ammonium carboxylate is represented bythe formula: R¹COONH₄ (wherein R¹ represents an alkyl group having 1 to4 carbon atoms) and the saturated aliphatic carboxylic acid amide isrepresented by the formula: R¹CONH₂ (wherein R¹ represents an alkylgroup having 1 to 4 carbon atoms).
 11. The process as claimed in claim10, wherein the saturated aliphatic carboxylic acid is acetic acid, thesaturated aliphatic ammonium carboxylate is ammonium acetate and thesaturated aliphatic carboxylic acid amide is acetamide.